Relative Minor

[mcbobie]

How do you tell what the relative minor of a major is. I'm talking about scales. Is there a chart or do you just have to memorize it?

[Musenfreund]

The relative minor is the sixth note of the major scale.

Or you can count back three frets on the guitar.

[hbriem]

The relative minor is built on the 6th degree of the major scale (1 whole step higher than the 5th if that helps).  
In C major, A is the 6th, 2 frets higher than the 5th, G.  

Backwards, the major is the flat 3rd of the minor.  In A major, C is the b3, 3 frets up.

It becomes 2nd nature eventually.  

Look at this partial neck diagram.  
M is the major, m is the relative minor (6th), 5 is the 5th (of the major).

[tt]
--|-----|-----|--m--|-----|-----|--M--|-----|-----|
--|-----|-----|-----|-----|-----|--5--|-----|--m--|
--|--m--|-----|-----|--M--|-----|-----|-----|-----|
--|-----|-----|-----|--5--|-----|--m--|-----|-----|
[/tt]